Not able to release PO through ME29N

Hi MM experts,

PO release strategy configured correctly and release strategy tab also coming in the PO header with correct release codes. But while going for ME29N for releasing the PO, it is not possible. (the green tick near the yellow triangle is not coming; only yellow triangle (release possible) is appearing). It gives the warning message "PO document 7100000027 cannot be released". Pl guide me how to solve the issue. screen shot given below.